In the case of one known telecommunication network, the terminal units are connected to the switch by means of a number of links. Various means, such as subscriber lines, tone-code receiving circuits, ring generators, etc., are connected to the terminal units. A regional processor monitors several terminal units and is connected to said units via separate signal lines. Another regional processor monitors other terminal units and is connected with these units via separate signal lines. Thus, the system includes a number of regional processors which monitor the activity of the terminal units. A regional processor also monitors the activity of the switch. The regional processors are connected by means of a signal bus line to a main central processor which controls the telecommunication network and which also controls the establishment of a connection through the switch on the basis of a subscriber-number list stored in the memory of the central processor. When a connection is to be established between two subscribers, line signalling is effected via the aforesaid signal lines, also called signal links, whereas the actual transfer of information, i.e. speech in the case of a speech connection or the transmission of digital data when two computers communicate with each other, is effected via those links which enter the switch from respective terminal units.
Each terminal unit has a local processor which monitors the activity of the devices or means connected to the terminal unit. When one device in a terminal unit wishes to establish a connection through the switch to a device in another terminal unit, the local processor signals a request to be connected to its regional processor, which in turn signals the request to the central processor, which carries out a number analyses and assigns idle multiple positions in the switch to the two devices.
One drawback with this known telecommunication system is that separate signal links are required downstream of the line terminating circuits. Communication equipment for signalling over the signal links is also required, in addition to the signal links. The signal links are expensive in themselves and require separate maintenance. Those links over which information is transmitted must also have their own communication equipment and also need to be maintained individually. The two information-transmission and line-signalling communication systems must be adapted so that they will function together.
When using known circuit switches, it takes a relatively long time to establish and release, or terminate, a connection. It can be mentioned by way of example that the time taken to establish a connection and to terminate said connection are each in the order of 10 milliseconds. This is disadvantageous when only a small amount of information is to be transmitted over the circuit-switched connection. When the connection is established between two processors and one of the processors is intended merely to send a confirmation signal to the other processor, the time taken to set-up the connection and to terminate the connection is relatively long in relation to the connection data phase, i.e. the time taken to transmit the information from one processor to the other.
Another drawback with the known switch is that the devices and terminal units connected thereto are allocated a fixed bandwidth which cannot be changed without undertaking comprehensive modification of the terminal units.